In general, a lithium secondary battery has a structure in which a lithium electrolyte is impregnated into a battery assembly including a cathode containing a transition metal oxide as an electrode active material, an anode containing a carbon based active material, and a separator. The lithium secondary battery as described above has a non-aqueous composition, and in general, an electrode is manufactured by coating electrode slurry on a current collector. For instance, the electrode slurry is prepared by mixing an electrode mixture including an electrode active material for storing energy, a conductive material for imparting electrical conductivity, and a binder for adhering the electrode active material and the conductive material to the current collector and providing binding strength therebetween with a solvent such as N-methyl pyrrolidone (NMP), or the like. As a current collector of a secondary battery, copper foil, aluminum foil, or the like, is generally used.
However, dust, or the like, may be generated due to deterioration of adhesion between the electrode mixture and the current collector in a compression process or subsequent manufacturing process at the time of manufacturing the battery as described above, and the electrode active material attached to a surface can be delaminated in operation of the battery. Deterioration of adhesion and delamination of the active material in accordance therewith as described above increase internal resistance of the battery to deteriorate output characteristics, decrease capacity of the battery, and the like, thereby significantly deteriorating performance of the battery.
Therefore, in order to solve this problem, various methods have been suggested. For example, a method of increasing binding strength with a current collector by etching a surface of an aluminum current collector to form micro unevenness has been reported. This method has an advantage in that it is possible to obtain an aluminum current collector having a high specific surface area by a simple process, but there is a problem in that a cycle life of the aluminum current collector is decreased due to etching treatment.
One of the main causes of generating a delamination phenomenon of a cathode active material in a cathode using a cheap aluminum current collector is formation of a coating such as an aluminum fluoride (AlF) coating, or the like, on a surface of the current collector due to a reaction between a fluorine source of an electrolyte and aluminum of the current collector in an operation voltage of the cathode. Formation of the AlF coating as described above can be accelerated due to an increase in the fluorine source at the time of increasing a temperature of the battery. The AlF coating deteriorates binding strength between the cathode active material and the aluminum current collector, thereby serving to increase resistance of the cathode.
Therefore, it was confirmed that the AlF coating causes delamination of the cathode active material and deteriorates electrical properties of the battery, particularly, a movement speed of electrons from the cathode active material to the current collect, thereby having a negative influence on performance of the battery.
Meanwhile, since an electrolyte solution containing a combustible organic solvent is used in the lithium secondary battery, a severe safety problem may occur at the time of various external impacts and creating a cell-uncontrollable environment, and there is a need to separately use an additional material for improving safety or to mount an additional safety device, in addition to a basic structure of a battery cell.
Therefore, an all-solid state battery in which a solid electrolyte is stacked between a cathode and an anode instead of an organic electrolyte solution and other components are configured in a solid state has been developed.
The all-solid state battery as described above has been spotlighted as a next-generation battery capable of basically solving the safety problems as described above because the organic electrolyte solution is replaced with a solid electrolyte.
Meanwhile, in the all-solid state battery, it is important to compress the solid electrolyte to have a high density and to allow interfaces to face each other without gaps therebetween. The solid electrolyte may have a small area, but in the case of attempting to increase an area of the solid electrolyte, it may also be difficult to uniformly compress the solid electrolyte.
Further, the solid electrolyte has been manufactured using a wet process in order to mass-produce the all-solid state battery, but in this case, when a thick film is coated on a composite electrode (a cathode composite electrode or anode composite electrode), the thick film may not be formed to have a uniform amount and thickness.
Further, since empty spaces (pores) are formed due to shapes of the solid electrolyte and active material powders, movement of ions are impossible, such that there is a problem in performance.
The foregoing is intended merely to aid in the understanding of the background of the present invention, and is not intended to mean that the present invention falls within the purview of the related art that is already known to those skilled in the art.